1. Field of the Invention
This invention pertains to the field of measuring and testing apparatus. More particularly, this invention pertains to the field of calorimeters. In still greater particularity, this device pertains to an adiabatic calorimeter to measure the electromagnetic wave energy absorptance of a sample material.
2. Description of Prior Art
In general, calorimeters are used to make thermal measurements. Different types of calorimeters are used to make specific types of measurements. A discussion of different types of calorimeters can be found in A New Dictionary of Physics by Gray and Isaacs, second edition, 1975, Longman Group Limited, London.
For low temperature thermal measurements an adiabatic vacuum calorimeter was developed by Simon and Lange. A discussion of this calorimeter may be found on page 491 of A New Dictionary of Physics. In this device the calorimeter is suspended inside an enclosure. The calorimeter and the enclosure are placed in an evacuated container which is itself immersed in liquid hydrogen contained in a closed dewar vessel. The temperature of the liquid hydrogen may be varied by varying the pressure in the dewar vessel. For a determination of the specific heat of liquid hydrogen, the liquid hydrogen in the dewar vessel is boiled under reduced pressure until the hydrogen in the calorimeter is liquified. The enclosure is then evacuated and a current passed through a heater in the calorimeter. The liquid hydrogen vaporizes and is collected in a reservoir; and the mass of liquid vaporized being is deduced from the pressure and volume of the reservoir and the equation of state for hydrogen.
Calorimeters have also been used to measure laser energy. One such device is illustrated in U.S. Pat. No. 3,622,245 issued to Alvin L. Rasmussen on Nov. 23, 1971. In that device a mirror with a known specific heat capacity and known dimensions is irradiated with a laser beam until thermal equilibrium is achieved. The laser beam is reflected to a second mirror and then out of the apparatus. Temperature sensors are connected to each mirror and measure the energy absorbed by each mirror. The device also has an optional heater which may be positioned in the mirror as a calibration device. This type of calorimeter is a steady state calorimeter, that is, the energy beam heats the material until a steady state (constant elevated temperature) is reached. As such, quantities such as heat capacity and heat diffusivity of the irradiated material must be known. Because the calorimeter is not adiabatic, there are temperature variations in the sample such that the same area must be irradiated for each measurement in order to get consistent results. In addition, the sensitivity of steady state type calorimeters is less than adiabatic calorimeters such that low power sources cannot be used and small absorption measurements cannot be made.
Another steady state calorimeter is shown in U.S. Pat. No. 4,019,381 issued to Frank J. Elmer on Jan. 12, 1976. That device uses optical elements of uniform thickness supported by a mounting structure. The mounting structure has a small hole to allow electromagnetic energy to enter. Thermocouples are attached to the optical element and to the mounting structure. The energy absorbed by the optical element produces a temperature rise which is proportional to the power remaining in the beam after it passes through the optical element. This device has the same limitations as that previously described in that it is rate dependent, does not allow for scanning, and does not provide for low level optical measurement.
While the above-described devices are satisfactory for their intended purposes, a device having the capability to measure low level energy absorptance and, therefore, possessing greater accuracy, would be desirable. It would also be desirable to have sample scanning capability and to have a means for calibration in order that quantities such as the sample dimensions, heat capacity, and heat diffusivity need not be known.